Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of valuable products such as fuels and other chemicals. Cellulosic and lignocellulosic feedstocks and wastes, composed of carbohydrate polymers comprising cellulose, hemicellulose, and lignin are generally treated by a variety of chemical, mechanical and enzymatic means to release primarily hexose and pentose sugars, which can then be fermented to useful products.
Pretreatment methods are used to make the carbohydrate polymers of cellulosic and lignocellulosic materials more readily available to saccharification enzymes. Standard pretreatment methods have historically utilized primarily strong acids at high temperatures; however due to high energy costs, high equipment costs, high pretreatment catalyst recovery costs and incompatibility with saccharification enzymes, alternative methods are being developed, such as enzymatic pretreatment, or the use of acid or base at milder temperatures where decreased hydrolysis of biomass carbohydrate polymers occurs during pretreatment, requiring improved enzyme systems to saccharify both cellulose and hemicellulose.
Teixeira, L., et al. (Appl. Biochem. and Biotech. (1999) 77-79:19-34) disclosed a series of biomass pretreatments using stoichiometric amounts of sodium hydroxide and ammonium hydroxide, with very low biomass concentration. The ratio of solution to biomass is 14:1.
Elshafei, A. et al. (Bioresource Tech. (1991) 35:73-80) examined the pretreatment of corn stover utilizing NaOH.
Kim, T. and Y. Lee (Bioresource Technology (2005) 96:2007-2013) report the use of high amounts of aqueous ammonia for the pretreatment of corn stover.
Intl. Pat. App. Publ. No. WO2004/081185 discusses methods for hydrolyzing lignocellulose, comprising contacting the lignocellulose with a chemical; the chemical may be a base, such as sodium carbonate or potassium hydroxide, at a pH of about 9 to about 14, under moderate conditions of temperature, pressure and pH.
U.S. Pat. Nos. 5,916,780 and 6,090,595, describe a pretreatment process wherein a specified ratio of arabinoxylan to total nonstarch polysaccharides (AX/NSP) is assessed and used to select the feedstock.
Most pretreatments such as the ones described above either result in a pretreated biomass depleted of lignin and hemicellulose or the partial depletion of hemicellulose with retention of most of the lignin. Therefore a method is needed to selectively remove only lignin without significant loss of either hemicellulose or cellulose from the biomass, as these constitute the source of sugars for fermentation. Methods are also needed to convert the delignified biomass to monosaccharides (sugars) economically and in high yield.
A number of pretreatment methods utilizing phosphoric acid or sulfuric acid have been disclosed. For example, U.S. Pat. No. 4,058,411 discloses a process for decrystallizing cellulose in natural cellulosic material through the use of concentrated H3PO4, which is extracted and recycled. The H3PO4 ranges in concentration from 80 weight percent to 85 weight percent.
U.S. Pat. No. 4,645,658 discloses a method of recovering concentrated hydrochloric acid from the product obtained from the acid hydrolysis of a cellulose containing material such as biomass. The method involves contacting such product with an extracting solvent.
U.S. Pat. No. 5,188,673 discloses a single step method of converting lignocellulosic materials to sugars including combining and mixing a low solids content lignocellulosic material with concentrated sulfuric acid. A modified single step method includes dilution of the reaction product with water, followed by continued reaction and subsequent separation of the sulfuric acid and sugar solution.
U.S. Pat. No. 5,486,068 discloses a process for treating wood, wood wastes, paper, and/or other types of polysaccharides (matter composed essentially of cellulose and lignin) with concentrated sulfuric acid (93 to 98.5% H2SO4), or with concentrated phosphoric acid (75 to 85% H3PO4), or various mixtures of these acids in their concentrated forms. The product of the process, dry solid compositions of matter, can be used for treating agricultural soils or as a landfill.
U.S. Pat. Nos. 5,417,984 and 5,674,507 disclose a method to prepare low crystallinity cellulose by reacting cellulose materials with 85% or higher weight percentage phosphoric acid under controlled sequenced temperature conditions.
U.S. Pat. No. 5,726,046 discloses a method for producing sugars using concentrated acid hydrolysis of biomass. The cellulose and hemicellulose in the biomass is first decrystallized and then hydrolyzed to produce a hydrolyzate containing both sugars and acid. Use of hydrochloric, hydrofluoric, phosphoric, and sulfuric acids is disclosed.
Intl. Pat. Appl. Pub. No. WO 02/02826 describes a process for production of fermentable sugar from cellulose-containing raw materials which are hydrolyzed with an acid-containing solution, particularly comprising sulfuric acid, whereupon the acid is removed from the mixture with an extraction agent. The method is characterized in that a mixture of a lower alcohol and a lower ketone is used as extraction agent.
Intl. Pat. Appl. Pub. No. WO 2007/111605 describes a process and system for the efficient fractionation of lignocellulosic biomass into cellulose, hemicellulose, sugars, lignin, and acetic acid. One step of the process combines a first solvent with the lignocellulosic biomass; another process step combines a second solvent with the material from the previous step. The first solvent comprises one or more chemicals selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, polyphosphoric acid, acetic acid, sulfur dioxide, zinc chloride, sodium hydroxide, potassium hydroxide, ammonia, among other chemicals or chemical combinations.
Improved, economical methods for decrystallization of cellulose in biomass are needed. Efficient decrystallization of cellulose in biomass and the ability to provide a higher loading of carbohydrates available for hydrolysis (saccharification) are advantageous for a commercial process for the production of fermentable sugars from a renewable resource biomass. Such advantages can make a process to provide high yields of sugars at high concentrations economically competitive.